1. Field of the Invention
The present invention relates to an electric discharge machining apparatus for machining a workpiece by applying voltage between a tool electrode and a workpiece to cause an electric discharge in between, and in particular to detection of the electric discharge in a case where an electric discharge energy is relatively low, such as in finish machining.
2. Description of Related Art
The electric discharge machining apparatus performs machining by electric discharge produced by applying voltage between a tool such as a wire serving as one electrode and a workpiece serving as the other electrode. In the electric discharge machining, it is necessary to make relative motion of the tool electrode and the workpiece such that a distance between the electrodes is kept constant. There has been generally carried out an average voltage servo in which an average machining voltage between the electrodes is detected and the tool electrode is feed relative to the workpiece such that the average machining voltage is constant. However, the average machining voltage varies in dependence on degree of contamination of machining fluid intervened between the electrodes and does not precisely reflect the distance between the electrodes and thus the average voltage servo has a problem in feed control for machining of high precision.
Therefore, it has been proposed to detect individual electric discharges and controlling the machining feed in accordance with the number of electric discharges (see JP 2004-283968A and JP2002-254250A).
Meanwhile, there has been carried out a method in the finish machining where stray capacitance in electric discharge cables, etc. is charged without supplying the machining current directly from a machining power source and the machining is performed by discharging an electric current from the stray capacitance so as to restrict a machining energy to a minimum value.
FIG. 1 shows an example of a power supply circuit for performing machining by discharging energy charged in the stray capacitance to be applied between the tool electrode and the workpiece.
In FIG. 1, E denotes a direct current power source, SW denotes a switching element, R denotes a current restriction element such as a resistor, 1 denotes a tool such as a wire serving as one electrode, 2 denotes a workpiece serving as the other electrode, C denotes a stray capacitance produced between the two electrodes by power cables connected to the tool electrode 1 and the workpiece 2, etc. The switching element SW is periodically turned ON/OFF to charge the stray capacitance by electric current from the power source E through the current restricting element R, and the charged voltage is applied between the electrodes to cause electric discharge in between. Since the switching element SW is periodically turned ON and OFF, it is prevented that arc discharge maintains by the charging current from the power source E when the electric discharge occurs between the electrodes.
FIG. 2 shows an operation of the power supply circuit shown in FIG. 1.
The switching element SW repeats ON/OFF periodically and while the switching element SW is turned ON, the stray capacitance C between the electrodes is charged to raise a voltage Vg between the electrodes. While the switching element SW is turned OFF, the voltage Vg between the electrodes decreases due to a leakage current between the electrodes. Thus, the stray capacitance C is gradually charged by the periodic ON/OFF of the switching element SW to raise a charged voltage and thus the voltage Vg between the electrodes. Then, an electric discharge occurs between the tool electrode 1 and the workpiece 2 to flow an electric discharge current by a discharge of the energy accumulated in the stray capacitor so that the voltage Vg between the electrodes is lowered. The above operations are repeatedly performed such that the voltage Vg between the electrodes in the form of saw-tooth is produced.
Since the inter-electrode voltage Vg is quickly lowered to an arc voltage when an electric discharge occurs between the electrodes, in a case of rough machining where the constant machining voltage is applied between the electrodes until occurrence of the electric discharge, the electric discharge is detected by monitoring the voltage Vg between the electrodes based on comparison of the detected voltage Vg with a reference value. However, in the finish machining utilizing the stray capacitance so as to restrict the machining energy to a minimum value, the voltage Vg between the electrodes varies in the form of saw-tooth and a value of the voltage Vg at the occurrence of the electric discharge is not constant, it is difficult to detect the occurrence of the electric discharge based on the drop of the voltage Vg between the electrodes.
Therefore, it is considered to detect the electric discharge based on the electric discharge current, as shown in JP2002-254250A. In general, there is a difference more than one order between the charging current and the leakage current of the stray capacitance and the electric discharge current, and therefore it is possible to detect the electric discharge based on the electric discharge current.
In order to detect a high-frequency current of the electric discharge, there are generally known a hollow coil such as the Rogowskii coil and a coil with a core such as a ferrite core in which a greater gain can be get. Since the above coils detect the electric discharge current in the form of the AC coupling, an offset component having an average value of zero is superposed on a measured value to make difficulty in determining occurrence of the electric discharge by the low-level current.
FIG. 6 shows a result of measurement of the current between the electrodes by the sensor coil. In FIG. 6, the graph 6A shows a voltage Vg between the electrodes and the graph 6B shows a measured value (after integration) of the current between the electrodes by the sensor coil. An axis of abscissa represents time having a division of 20.0 μs and an axis of ordinate represents the voltage between electrodes having a division of 20.0 V (in the upper graph) and the current between electrodes having a division of 100 mV (in the lower graph). As shown in the graph 6B, a measured value of current between electrodes contains an offset component of low-frequency interposed therein and detected as being waving. In the example shown in FIG. 6, positive voltages and negative voltages are alternately applied between the electrodes so as to prevent electric corrosion in the case of using machining fluid of electrolyte such as water.
In the measurement of the current between the electrodes by the coil, it is difficult to detect the electric discharge by the current of low level since the offset component is superposed in the measured value due to the AC coupling, as described above. On the other hand, a current measuring device using the Hall element for detecting the electric discharge current in the form of a DC coupling does not have the above problem but is not suitable for the detection of high frequency current in the finish machining since it has a low response characteristic.